1. Field of the Invention
The present invention relates to a knock control apparatus for an internal combustion engine which forms pulses of vibration components superposed on an ion current, and performs knock control on the internal combustion engine in accordance with the number of pulses. More specifically, the present invention relates to a knock control apparatus capable of performing stable knock control in the event that the number of pulses generated changes according to external factors (e.g., fuel, soiled spark plugs, etc.).
2. Description of the Related Art
It is known that with internal combustion engines, ions are generated by fuel combustion. Hence, these ions can be detected as an ion current by providing probes to which high voltage is applied within each cylinder of the engine. Further, it is known that a vibration component of the same vibration frequency as knocking is superposed onto this ion current, and this vibration component is extracted so as to perform knock control.
With general knock control apparatuses using ion current, a high voltage for detecting an ion current is generated using a secondary voltage generated by an ignition coil, and the high voltage is applied to spark plugs in cylinders of an internal combustion engine following discharge for ignition, thereby measuring the ion current. The vibration component of the knock frequency band is extracted from the ion current by a band-pass filter (BPF), amplified by an amplifier, and waveform shaped by comparison with a detecting threshold value to thereby generate knock pulses.
FIG. 10 illustrates the ion current waveform in the event that there is no knocking, the amplified vibration component, and knock pulses. FIG. 11 illustrates the waveform in the event that knocking is occurring. It can thus be understood from these figures that the knocking state can be detected by the number of knock pulses.
The knock pulses are sent to an engine control unit (hereinafter referred to as an ECU), so that is is judged whether or not knocking is occurring, and control of the ignition timing, i.e., knock control is performed based on the judgement results.
Conventional knocking judgement has been made on the basis of an average value (AVE) of the knock pulse number (npn) counted by the ECU and a dead band (OFS). The average value (AVE) of the knock pulse number is a learned value calculated from the running average of the knock pulse number (npn) for each cylinder, and is represented by the following expression.
Running average: AVE=AVE+(npnxe2x88x92AVE)xc3x97Kflt
wherein Kflt is a correction coefficient, and is a value between 0 and 1.
On the other hand, the dead band (OFS) is a variable for absorbing irregularities in the knock pulse number (npn), and is a map value with the revolutions/minute and load of the internal combustion engine as parameters. Accordingly, the background level (BGL) is calculated for each cylinder using the following expression, thus performing knocking judgement using this background level.
BGL=AVE+OFS
FIG. 9 illustrates the transition in npn, AVE, and BGL in a state without knocking, obtained from an actual internal combustion engine. As shown in FIG. 9, large pulses occur intermittently, so the BGL makes transition at an extremely low level if there is only AVE. Accordingly, unnecessary retardation (noise retardation) occurs in the event that npn greater than BGL despite there being no knocking, consequently leading to loss of power of the engine. An OFS is set in order to prevent this from occurring, but as shown in FIG. 9, there is the tendency that the OFS becomes the dominant factor in determining the BGL.
The knock intensity (np) is represented by the following expression, and the retardation amount of the ignition timing is determined according to the np.
np=npnxe2x88x92BGL (with restrictions of npxe2x89xa70)
In the event that npxe2x89xa71, i.e., when it is judged that knocking is occurring, the system switches Kf it which is used for calculating the AVE, thereby suppressing unnecessary rising of the BGL. Further, even in the case of np less than 1, Kf It is switched depending on whether npnxe2x89xa7AVE or npn less than AVE.
Also, the retardation amount has a holding time with the revolutions/minute and load of the internal combustion engine as parameters, and in the event that retardation is not generated within this holding time anew, the retardation amount is decreased at a predetermined speed. In the event that new retardation is generated within the holding time, the holding time starts from that point.
Rapid changes in the driving conditions of the engine are generally judged based on the rate of change in the degree of opening of the throttle, and the rate of change in revolutions/minute. In the event that the ECU judges that a change more rapid than these conditions has occurred (transient state), the OFS map switches to a map for transition for a predetermined period. This is performed in order to keep the change in knock pulses generated by changes in operating conditions from being misjudged as knocking, and also to improve the detection of knocking occurring during transition.
With the above-described conventional method, the maximum value of AVE is npn even in the event that the follow-up of the AVE is maximized, and in order to avoid noise retardation due to irregularities in npn, the OFS must be more than the irregularities in npn.
However, in the event that the number of times of generation of npn changes due to the type of fuel or spark plugs, effects of soiled spark plugs, changes over time in the internal combustion engine, and so forth, the appropriate OFS changes as a matter of course, and accordingly there has been the problem that this method cannot deal with changes in the npn owing to external factors. In the event that the appropriate OFS becomes greater than the set value due to this factors, i.e., in the event that OFS is insufficient, the BGL approximates the npn due to the learning effect of the AVE, but the knock judging conditions switch Kflt at npxe2x89xa71, so there has been the following problems: judgement that knocking is occurring is easily made even though within average irregularity ranges;
and retardation in ignition timing is generated and reaches the maximum value even though not in a state of knocking.
That is, even in the event that Kflt is changed to improve the follow-up of AVE, the maximum value of the AVE is npn and in order to prevent noise retardation due to irregularities in the number of knock pulses there is the need to have the OFS greater than the amount of irregularities, but the irregularities change according to various conditions.
Also, there is the problem that in the event that the OFS is set at the maximum irregularities for all cylinders due to the above reasons, if there is a state with few knock pulses or a cylinder with few knock pulses, detection of knocking, which is in fact occurring, becomes impossible.
Accordingly, the present invention is intended to solve the above problems, for example, by performing knock judgment based on a maximum value (MAX) of the number of knock pulses (npn).
Bearing the above in mind, according to the present invention, there is provided a knock control apparatus for an internal combustion engine comprising: a knock detector for extracting a vibration component superposed on an ion current, and waveform shaping it at a predetermined threshold value to generate a train of knock pulses (Kp) of which the number of pulses indicates an intensity of a knock generated in the internal combustion engine; and an engine control unit for counting the number of knock pulses (npn) in the knock pulse train (Kp) output from the knock detector, and judging, based on the knock pulse number (npn), whether or not a knock is occurring; wherein the engine control unit performs a peak hold of the knock pulse number to calculate the knock judgement threshold value (BGL) by multiplying the number of pulses by (xcex1).
Preferably, the knock judgement threshold value (BGL) is calculated by the following expression:
BGL=xcex1xc3x97MAX+xcex2
where xcex1and xcex2are constants and MAX is a maximum value for the number of knock pulses (npn) in the knock pulse train (kp).
Also, MAX is updated for each combustion cycle, and in the event that the number of knock pulses (npn) currently generated is equal to or greater than a current value for MAX, the updating is performed according to the following expression:
MAX=MAXxc3x97xcex51+(npnxe2x88x92MAX)xc3x97xcex52
where xcex51 and xcex52 are correction coefficients.
Further, in the event that combustion cycles continue with the number of knock pulses (npn) being equal to or smaller than the MAX value, the MAX value is held for a predetermined period of time, and the MAX value is decreased at a predetermined speed (xcex3) after a lapse of the predetermined period of time.
Preferably, the engine control unit switches over at least one of the correction coefficients, depending on whether or not a S/N ratio (npn/BGL) represented by a ratio of the number of knock pulses (npn) and the knock judgement threshold value (BGL) is equal to or greater than a predetermined value (xcex4).
Preferably, the engine control unit detects a knock in the event that the number of knock pulses (npn) is greater than the knock judgement threshold value (BGL) (npn greater than BGL), and generates an amount of retardation by which ignition timing of the internal combustion engine is retarded.
Preferably, the engine control unit calculates the knock intensity np by the following expression:
np=npn/MAX;
or by the following expression:
np=npn/BGL;
and generates the amount of retardation according to the knock intensity np thus calculated.
Preferably, the engine control unit sets an upper limit for the number of knock pulses (npn).
Preferably, the engine control unit does not update the MAX value but maintains the current MAX value in the event that restriction is applied at the upper limit value of the number of knock pulses (npn), and the engine control unit prohibits knock control in the event that abnormal ignition cycles of a predetermined number or more have occurred within a predetermined number of ignition cycles (i.e., (npn) is equal to or greater than a predetermined restriction value).
Preferably, the engine control unit also changes the values of the correction coefficients (xcex51) and (xcex52), which can be switched over by the S/N ratio of knock pulses, by the amount of retardation generated in the ignition timing as well.
Preferably, in the event that judgement is made that an engine operating condition is changing rapidly, the engine control unit decreases or increases at least one of the correction coefficients xcex51 and xcex52, the MAX value decreasing speed (xcex3) and the coefficient (xcex1) over a predetermined period of time.
Preferably, in the event that judgement is made that an engine operating condition is in a transient state, the engine control unit judges whether the engine operating condition is changing in a direction in which the number of knock pulses increases or decreases, and switches over parameters for determining the increasing or decreasing speed of BGL over a predetermined period.
The above and other objects, features and advantages of the present invention will be more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments of the invention taken in conjunction with the accompanying drawings.